1. Field of the Invention
This invention relates to storage stable polyorganosiloxane compositions. More particularly, this invention relates to a method for preparing one-part storage stable organosiloxane compositions curable by a platinum catalyzed hydrosilation reaction. The viscosity and the cure time of curable compositions prepared in accordance with the present method do not increase substantially during storage.
2. Description of the Prior Art
Polyorganosiloxane compositions that cure by means of a platinum catalyzed hydrosilation reaction to form silicone gels are disclosed, for example, in U.S. Pat. No. 3,020,260, issued to Nelson on Feb. 6, 1962, and in U.S. Pat. No. 4,374,967, issued to Brown, Lee and Maxson on Feb. 22, 1983.
The gels described by Nelson are obtained by reacting an organosiloxane containing two silicon-bonded hydrogen atoms per molecule with a vinyl-containing copolymer wherein the repeating units are of the formulae RViSiO, R.sub.2 SiO, and CH.sub.3 R.sub.2 SiO.sub.0.5, R is methyl or phenyl and Vi represents vinyl.
The vinyl-containing copolymers disclosed by Brown et al. consist essentially of (CH.sub.3).sub.2 SiO, (CH.sub.3).sub.3 SiO.sub.0.5 and (CH.sub.3).sub.2 ViSiO.sub.0.5 units, where Vi represents vinyl, and are reacted with an organohydrogensiloxane having more than one silicon-bonded hydrogen atom per molecule. Brown et al. teach that known inhibitors for platinum catalyzed hydrosilation reactions can be added to increase the working time of catalyzed compositions by decreasing the rate at which these compositions cure at temperatures above about 100.degree. C.
While known inhibitors will partially suppress the reactivity of catalyzed compositions containing both vinyl radicals and silicon-bonded hydrogen atoms, these compositions cannot be stored for extended time periods under ambient conditions without either undergoing a substantial increase in viscosity or the time required to form a cured solid gel. The viscosity of commercially useful one-part curable organosiloxane composition should not vary substantially during the period of several weeks that can elapse between the time the composition is prepared and the time at which it is desired to cure the composition. During this period the compositions are stored in locations where the ambient temperature may reach 40.degree. C.
Inhibitors that have been disclosed as useful catalyst inhibitors for the compositions taught in the aforementioned Brown et al. patent include the aromatic hetrocyclic nitrogen compounds, pyridazine, pyrazine, quinoline, 2,2'-biquinoline, bipyridine, aphthyridine, quinaldine, dialkyl formamides, thioamides, alkylthioureas and ethylene thiourea described in U.S. Pat. No. 3,188,299, issued June 8, 1965, the organophosphorus compounds described in U.S. Pat. No. 3,188,300, issued June 8, 1965, benzotriazole as described in U.S. Pat. No. 3,192,181, issued June 29, 1965, the nitrile compounds described in U.S. Pat. No. 3,344,111, issued Sept. 26, 1967, the halocarbons described in U.S. Pat. No. 3,383,356, issued May 14, 1968, the acetylenic compounds described in U.S. Pat. No. 3,445,420, issued May 20, 1969, the vinyl silazanes described in U.S. Pat. No. 3,453,233, issued July 1, 1969, the sulfoxide compounds described in U.S. Pat. No. 3,453,234, issued July 1, 1969, the stannous salts, mercuric salts and other salts described in U.S. Pat. No. 3,532,649, issued Oct. 6, 1970 and the cyclic polymethylvinylsiloxanes discussed in U.S. Pat. No. 3,923,705, issued Dec. 2, 1975.
The use of ammonia, amines, and other nitrogen-containing compounds as inhibitors for the platinum catalyzed reaction of silicon-bonded hydrogen atoms with silicon-bonded hydroxyl groups is taught in U.S. Pat. No. 3,867,343, issued to Garden on Feb. 18, 1975. U.S. Pat. No. 4,281,093, issued to Garden on July 28, 1981, discloses using these inhibitors for The reaction of SiH-containing polyorganosiloxanes with polyorganosiloxanes containing silicon-bonded vinyl or allyl groups. The catalysts for the reaction are organic complexes of platinum or rhodium.
U.S. Pat. No. 4,584,631, which issued to Janik and Lo on Apr. 22, 1986, discloses storage-stable one-part heat curable polyorganosiloxane composition consisting essentially of The product obtained by blending (A) at least one organosiloxane copolymer consisting essentially of from 80 to 96.5 mol percent of (CH.sub.3).sub.2 SiO units, from 2.0 to 10.0 mol percent of CH.sub.3 SiO.sub.1.5 units, from 1.25 to 6.0 mol percent of (CH.sub.3).sub.3 SiO.sub.0.5 units and from 0.25 to 4.0 mol percent of (CH.sub.3).sub.2 (CH.sub.2 .dbd.CH)SiO.sub.0.5 units; (B) an organohydrogensiloxane containing at least two silicon-bonded hydrogen atoms per molecule, no more than one silicon-bonded hydrogen per silicon atom and silicon-bonded organic radicals selected from the group consisting of alkyl containing from 1 to 6 carbon atoms per radical, phenyl and 3,3,3-trifluoropropyl, said organohydrogensiloxane providing from 0.7 to 1.2 silicon-bonded hydrogen per silicon-bonded vinyl radical present in said composition, (C) a platinum catalyst in an amount sufficient to promote the reaction of (A) and (B) in the absence of an ihibitor, and (D) a platinum catalyst inhibitor.
These compositions are characterized by the presence of a platinum catalyst inhibitor of the general formula R.sup.1.sub.2 NR.sup.3 NR.sup.2.sub.2 or R.sup.4 NH.sub.2 in an amount sufficient to impart long term storage stability to said composition at 40.degree. C. without inhibiting the ability of the platinum catalyst to promote curing of the compositions at temperatures above about 80.degree. C. catalytic activity of (C) at 135.degree. C. In the foregoing formulae for the amines each R.sup.1 is individually selected from the group consisting of alkyl radicals containing from 1 to 4 carbon atoms, R.sup.2 is R.sup.1 or hydrogen, R.sup.3 represents an alkylene radical containing from 2 to 4 carbon atoms, and R.sup.4 represents an alkyl radical containing from 2 to 4 carbon atoms.
The curable compositions disclosed in the aforementioned Janik and Lo patent are prepared by blending together all of ingreidents, including the vinyl-containing polyorganosiloxane identified as ingredient (A). The amine prevents the resultant composition from curing when stored for several months at temperatures up to about 70.degree. C.
The present inventors found that when one-part curable compositions described in the Janik and Lo patent are prepared on a commercial scale by blending kilogram amounts of all the ingredients together using commercial scale mixing equipment the viscosity and the time required to cure the resultant curable compositions often increase substantially and unpredictably during storage at room temperature. These increases in viscosity and cure time are not observed when the curable compositions are prepared using laboratory scale equipment. The unpredictable variations in viscosity and cure time makes it difficult to prepare curable compositions with the consistant properties required for fabrication of commerically useful cured articles and gels from these compositions.